I want to know
depends on your luck with the catch rate formula, could be one, could be an infinite amount but the average comes out to be something like 30 i think. this page explanes catch rate.
it depends, i would suggest putting a status effect on it to make it easier to catch by the way (:
It all depends on the catch rate of Reshiram, 45. Using an Ultra Ball and lowering HP may cause a better chance of catching it but its all by a formula. You would have about a 34.9% of capture.